Ceramic packages are used for holding electronic devices, particularly microwave transistors and the like, and for providing readily available terminals for connection to other components.
A package for high power microwave devices must satisfy a series of seemingly inconsistent requirements. While it should be small and compact, it must be strong enough to withstand stresses during assembly, during connection to other packages, and during use. While the package must electrically insulate the device, it must, at the same time, conduct heat away with a minimum of thermal resistance. In addition, it must provide reliable electrical connection from the device to the outside world while sealing the device in its own environment.
In the packaging of high power microwave devices, the provision of means for heat removal is a particularly important consideration. Heat generated during the use of high frequency power transistors, for example, can lead to temperature buildup and premature failure.
A typical ceramic package for microwave devices comprises a base of heat conducting ceramic such as beryllium oxide or alumina, ceramic side walls, and a Kovar (nickel, iron, cobalt alloy) or ceramic cap. While two or more leads penetrate the side walls to the package interior where the device is mounted, the components are all brazed or soldered together to hermetically seal the device from the ambient atmosphere.
It has proven unexpectedly difficult to provide a safe, reliable means for removing heat from the ceramic base of such packages. The simple straight-forward approach of brazing the ceramic base to a copper flange has serious difficulties because, after repeated thermal cycling, the ceramic tends to break and chip, destroying the package seal. Attempts were made to overcome this problem by interposing between the copper and the ceramic, a "buffer" layer of material such as molybdenum which has a thermal coefficient of expansion closer to that of the ceramic than copper, but these layers were not effective in preventing cracking. As a consequence, a number of manufacturers have given up on copper flanges and turned to flanges of other materials, such as Elkonite (a copper, tungsten powdered metal), having less satisfactory thermal conductivity.